The packaging of integrated circuits, there are a various type of packaging methods and structures. For example, in a conventional Package-on-Package (POP) process, a top package is bonded to a bottom package. The top package and the bottom package may also have device dies packaged therein. By adopting the PoP process, the integration level of the packages may be increased.
In an existing PoP process, the bottom package is formed first, which includes a device die bonded to a package substrate. A molding compound is then molded on the package substrate, wherein the device die is molded in the molding compound. The package substrate further includes solder balls formed thereon, wherein the solder balls and the device die are on a same side of the package substrate. The solder balls are used for connecting the top package to the bottom package. Accordingly, the heights of the solder balls need to be greater than the thickness of the device die, so that the top portions of the solder balls may protrude above the top surface of the device die, and above the top surface of the molding compound. Accordingly, the lateral sizes of the solder balls are also large, and the amount of the solder balls that can be used in a PoP structure is limited.
In another type of packaging, which is known as possum packaging, a first device die is bonded to an overlying second device die. The second device die is bonded to a third device die that is underlying the first die and the second die. Since the bond structure for bonding the first device die and the second device die is misaligned with the bond structures of the second die and the third die, the footprint of the resulting package is large. On the other hand, the second die and the third die have a great distance therebetween since the first device die is disposed between them. The solder balls that bond the second die and the third die are thus large, causing the further increase in the footprint of the resulting package. This also results in the increase in the manufacturing cost.